Abstract

Quantitative measurements of fluid flow properties can be achieved by background-oriented schlieren (BOS). In this paper, it is shown that this depends on several factors. Image-quality index is used to investigate the influence of the image sensor and the quality of its output. Image evaluation is applied to synthetic images, which are treated with a step function, so that they simulate the sharp density jump. The gradual change of the evaluated vector shift revealed the major dependence on the interrogation window, and revealed less of a dependence on background features. BOS applied to shock-wave reflection from a wedge in a shock tube gave qualitative results, due to large uncertainties. But, the application to cooling by natural convection gave satisfactory results, comparable to thermocouple data and theory.

Figures (10)

Images (a) and (b)—the latter being the framed part in the former—are taken with Imacon DSR200 camera, and (c) with Shimadzu HPV-1. Images (a) and (c) show the same field of view, while (b) and (c) have the same pixel count. This detail of the entire image corresponds to the small rectangle starting at the 50th pixel in Fig. 2.

BOS result for the shock reflection from a wedge (black full lines), shown as a magnitude map of vector shift, with pixel values in coordinates and pixel shift amount in the magnitude bar. Contours border zones with 0.1 pixel shift difference.

BOS results for cooling by natural convection, taken at four different times: 10, 25, 63, and 105 s after the heater was turned off (times noted in the upper right corner of each map). TC positions are noted by the symbols inverted blocked triangle and blocked triangle. Coordinates give the distance from the center of the heat source, in millimeters, and the magnitude bar gives the pixel shift. Contour lines correspond to borders between zones with pixel shift difference of 0.5 pixels.